1. Field of the Invention
The present invention relates to control systems and methods and more particularly to control systems and methods useful where a system characteristic observed for control purposes can take values of the same sign and magnitude for both positive and negative variations of a variable that is adjusted to control the system. While aspects of this invention are believed to relate very generally to many different control systems and methods, aspects of the invention find their most immediate application to the positional control of detectors used in data acquisition systems. Specific embodiments of the invention are useful in servo mechanical control systems such as high density magnetic disk drives from which data are read by precisely positioning a magnetic read element adjacent to a set of predefined data storage locations.
2. Discussion of the Related Art
The related art is illustrated with reference to several simple control systems. A common task assigned to control systems is maintaining the relative position of one object with respect to another object, where both objects may be moving in an unpredictable manner. One useful system to consider is an optical disk player, schematically illustrated in FIG. 1, of the type that focuses laser light from a laser 1 on a data storage surface 2 of an optical disk to read out data from the disk. Typically the laser light diverges and is focused on the data storage surface 2 by objective lens 3. During operation, the disk may flex or vibrate so that the distance between the data storage surface 2 and the focal point of the objective lens 3 changes by enough to measurably degrade the focus of the laser light on the surface of the optical disk being read. To prevent signal variations and degradation, the disk player adjusts the position of the lens to maintain the separation between lens 3 and the data storage surface 2 near constant at the nominal “in focus” distance. Here and throughout the discussion of the background and the invention, the term nominal has its customary meaning as satisfactory or according to plan.
It is not typically practical to measure the distance between the lens 3 and the data storage system, so disk players indirectly observe this distance. For example, light reflected from the data storage surface may pass through a beam splitter 4, be collected and refocused by lens 5 and directed to an optical detector 6. The optical detector 6 is divided into four quadrants, as illustrated in FIG. 2. The system is designed so that, when the distance between the lens 3 and the data storage surface 2 is equal to the nominal “in focus” distance, the light incident on the detector 6 is in focus and has an intensity distribution that varies symmetrically on the surface of the detector. Such a symmetric, “in focus” state is illustrated in FIG. 3. Each quadrant of the detector provides a separate output voltage VA, VB, VC, VD, so the symmetric state of FIG. 3 is associated with an effectively zero value of the observable quantity (VA+VD)-(VB+VC).
Asymmetry is introduced into the laser light used to read the data storage surface 2 so that too short of a separation between the lens 3 and the data storage surface 2 produces an asymmetric, out of focus pattern on the detector 6 like that illustrated in FIG. 2. This asymmetry is characterized by high intensity light on quadrants B and C of detector 6 and low intensity light on quadrants A and D. Too long a separation between the lens 3 and the data storage surface 2 produces the out of focus pattern shown in FIG. 4, which is characterized by high intensity light on quadrants A and D of detector 6 and low intensity light on quadrants B and C. The amount of the asymmetry varies with the amount by which the separation between the lens 3 and the data storage surface 2 varies from its desired “in focus” distance. Consequently, the quantity (VA+VD)-(VB+VC) can be a useful variable to observe to control the position of the lens 3. For example, a linear control system can be provided to adjust the lateral position of the lens 3 to provide good control of the focus of the optical disk system of FIG. 1 using the quantity (VA+VD)-(VB+VC) as an input.
In the FIG. 1 system, the quantity (VA+VD)-(VB+VC) provides a good observable variable for controlling the position of lens 3 (the controlled variable) and hence the focus of the system (VA+VD)-(VB+VC) provides both a magnitude indicative of the extent of the necessary correction and the sign of the necessary correction. The sign of the necessary correction indicates whether the lens 3 is too close or too far away from the data storage surface 2 and so which lateral direction the lens 3 needs to be moved to optimize focus. (VA+VD)-(VB+VC) can be used to control the FIG. 1 system because (1) the laser light is purposefully made asymmetric within the system and (2) the detector is made up of four independent quadrant detectors. In other words, the optical system is especially adapted to allow for the easy control of the position of the lens 3.
While it is possible to control the system of FIG. 1 using the asymmetric beam in association with a quadrant detector, this is not an entirely desirable situation. It is easier to design optical systems for light that varies uniformly than it is to design similar quality optical systems for asymmetrically varying beams. This is particularly true when different colors of light might be used in the optical system.
Variations of the system of FIG. 1 that do not intentionally introduce asymmetry into the light beam could use a detector like that illustrated in FIG. 5, which includes an outer element 7 producing an intensity-dependent voltage output of VT and an inner element 9 producing an intensity-dependent voltage output of VC. Using such a system, the quantity VC/(VC+VT), schematically illustrated as a function of lens to storage surface separation S from the nominal “in focus” position SO, provides a measure of the focus of the system. It is possible to use this observable to adjust the focus of the system, but it is difficult. This is so because the quantity VC/(VC+VT) indicates a magnitude of a correction to be made, but does not provide a sign or direction for the correction. For any observed variation of the quantity VC/(VC+VT) from the peak, in focus value, the lens might need displacement in either a positive or a negative direction. It is thus difficult to directly control the FIG. 1 system using only the quantity VC/(VC+VT).
Systems like that indicated in FIG. 6 can be called absolute value systems, because the observed variable provides the absolute value of a correction to make, but does not indicate the sign or direction for the correction. For such absolute value systems, it is known to introduce a movement of known direction such as an oscillation to the controlled variable. This technique is known as dither or dithering. Using dither, a regular oscillation is introduced to the position of the lens 3, which introduces a regular variation in the observed quantity VC/(VC+VT). By comparing the phase of the oscillation in VC/(VC+VT) with the phase of the oscillations in the position of the lens 3, the control system can identify the direction of the correction to be made to the lens position. Consequently, dither allows for the mechanism illustrated in FIGS. 5 and 6 to be used to control the FIG. 1 optical disk player.
Dither has a variety of drawbacks. It is complex, requiring introduction of a detectable amount of motion between objects. Moreover, dither is itself a noise source, and so is generally undesirable. Consequently, it is more common to design a system to have an observable variable that provides both a magnitude and direction or sign for a correction than it is to use a system like that illustrated in FIGS. 5 and 6.
The above example relates to the control of the optics of an optical disk player and illustrates the difficulty of trying to control such a system using an absolute value observable variable. In this sense, an absolute value observable variable is one that varies with the variable to be controlled, but only in magnitude as illustrated by the graph of FIG. 6. A similar problem can be illustrated with reference to magnetic disk storage systems and this discussion now turns to that technology. This background discussion illustrates aspects of the problem to which aspects of the present invention are addressed as well as aspects of an environment in which particularly preferred aspects of the present invention are implemented.
Magnetic disk drive data storage systems provide high volume, long term data storage that is comparatively fast and relatively inexpensive, at least as measured on a per-bit basis. For example, magnetic disk storage is faster than present optical storage options and is comparatively less expensive than present flash memory based storage devices. Industry today relies on magnetic disk drives for long term data storage in various types of computer systems and in certain consumer electronics applications such as video recording and playback, and both types of uses continue to grow. Research into magnetic storage disk systems continues and the performance of such systems is expected to continue to improve.
Rotational storage devices and in particular disk drives store data on one or more faces of a rotating media, often referred to as platters or disks. In the case of a conventional hard disk drive, data are stored by generating a magnetic modulation within a magnetic material coated on a data storage surface of the disk. Data are read back by subsequently detecting this modulation with a read head. Typically data are written to a disk using a write element and data are read from the disk using a read element, where both the write and read elements are provided as physically distinct elements on a single head. By recording data in the form of magnetic signals on the rotating disk, data can both be stored and subsequently recovered after even long periods of time. Data may be organized into a plurality of radially displaced, tangentially extending tracks, with the data stored on the tracks generally organized into a plurality of data blocks. To read or write data from or to a particular data block, the typical disk drive positions the read and write head over the track containing the target data block in what is known as a seek operation. The read and write head of the disk drive then reads or writes the data on the storage surface, as desired. Data read and write operations, seek operations and other operations such as using Grey codes to identify track positions are described in U.S. Pat. Nos. 5,523,902, 5,796, 543, and 5,847,894, each of which is hereby incorporated by reference.
FIG. 7 illustrates schematically certain aspects of a storage surface 10 and of a disk drive. Generally, the disk 8 includes a central opening 12 through which passes the spindle of the disk drive and by which the disk and its storage surface are rotated. An area 14 is provided on the disk around the central opening 12 for clamping or otherwise holding the disk to the spindle; this area 14 is essentially unusable for storage. The storage surface 10 extends radially away from the clamping area 14 and may terminate in a peripheral band of the disk, not shown, that is also preferably not used for data storage. In general, the storage surfaces of disk drives may be considered substantially uniform or the storage surface might be divided into plural zones. When the storage surface 10 is treated as substantially uniform, relatively little is done in the disk drive to account for the differences between radially displaced data storage locations such as differing rotational velocities and the associated differences in the areal density of stored data on the disk surface. Another strategy subdivides the storage surface 10 into a number of radially extending bands, known as zones, such as the zones 16, 18 and 20 indicated on the storage surface 10. The various storage locations within the different zones 16, 18 and 20 are treated similarly, while the storage locations in different zones may be treated differently, for example by using different clock rates for reading or writing different signals or by using different densities of servo information.
The read and write head 22 is a small assembly provided on the end of an arm or transducer assembly 24 that moves the head 22 over the storage surface 10. The transducer assembly may move the head 22 by rotation, by translation or by a combination of rotations and translations. For example, many present drives provide larger movements by rotating the transducer assembly about a pivot on the end of the transducer assembly opposite that of the head 22. Additional adjustments may be accomplished using fine translations, which might be accomplished, for example, using piezo-electric elements. In general, the mechanical rotational and translational movements of the head 22 are preferably accomplished under servo control using, for example, voice coil motors or other compact, fast response systems. The read and write head 22 of the transducer assembly is typically not rigidly attached to the transducer assembly. Rather, the read and write head is preferably mounted on a slider coupled to the transducer assembly through a flexible assembly. Typically the slider is designed to “fly” on an air bearing over the data storage surface created between the shaped undercarriage of the slider and the disk.
FIG. 8 illustrates in greater detail aspects of one currently favored read and write head design. The illustrated read and write head 22 is mounted on one end of a slider 26 that is, in turn, mounted to the transducer assembly (not shown in FIG. 8). A magnetoresistive read element 28 is formed as a thin film element near or on the end surface of the slider and then an inductive or other type of write element 30 is provided partially over the read element 28. A protective coating 32 covers the read and write head 22. As illustrated, it is typical that the write element 30 is considerably larger (sometimes 160% or more) than the read element 28. In addition, the read element 28 is typically offset to one side with respect to the write element. This configuration is characteristic of the illustrated types of elements and causes the read and write head to have different preferred positions with respect to a track or other data storage structure for respective read and write operations.
A read and write head 22 associated with a storage surface is precisely positioned with respect to data storage locations along a track through the use of servo control mechanisms within the disk drive that operate in conjunction with positional servo information stored on the storage surface of the disk drive. Various servo schemes have been used historically for magnetic storage disk drives, with the industry presently preferring the use of servo information included on each data storage surface on the disks within the disk drive. In reliably performing a track seeking operation, the disk drive uses the read element 28 of the head to detect servo position information that is used by control circuitry to position the transducer assembly and the head over the target track. The servo position information identifies the position of each track and provides at least a relative identification for each of the tracks on the disk drive.
Positional control or servo information most often is stored within radially extending sector servo wedges, described in greater detail in the above-referenced patents, precisely placed on the disk's data storage surface during the original manufacture of the disk storage device. The positional and other servo information may be written with a servo writer like that described in U.S. Pat. No. 4,920,442 or in accordance with the methods described therein. Servo writers are used in a factory initialization process to write positional and other servo information on the storage surfaces of the disks, along with other information to prepare the storage surface for use. The servo writer, typically using precise positional information provided by a laser positioning mechanism, most often places servo information on each track along predefined radial spokes, defining the beginning of each sector on the disk.
FIG. 7 shows two possible organizations of servo information on the storage surface of a disk 8, the one discussed above in which full radial wedges 34 extend over the usable radial extent of the data storage surface 10 and another in which partial servo wedges 36 are provided in different densities in different zones of a storage surface. In the first method there may be on the order of 100-200 servo bursts positioned at regular angular intervals on the storage surface of a 3.5″ storage disk. Different designs and operational parameters can change these characteristics significantly. In the second of the methods, there is an increasing number of servo wedges in each of the zones as they progress away from the center of the disk. Generally, only one of the two schematically illustrated methods is used on a disk.
Regardless of whether zones are differentiated on the data storage surface, the servo wedges may includes a significant amount of information useful for positioning the head and for reading and writing data to the disk. An illustration of the information that may be included in the servo wedge is provided, for example, in previously incorporated by reference U.S. Pat. No. 5,796,543 and is reproduced in FIGS. 9A & 9B, which respectively illustrate writing and reading operations. These figures show a portion of a servo wedge 40 where it extends across four data tracks Tr 0, Tr 1, Tr 2 and Tr 3. The wedge 40 is made up of a servo preamble 42 and servo position information 44 and the wedge is followed by one or more data blocks 46 in each of the tracks. A head 22 including both read and write elements is shown to indicate the procession of data (leftward) by the head and the preferred position of the head with respect to the centerline of the track during writing (FIG. 9A) and reading (FIG. 9B) operations. As shown, the read elements is typically maintained off the center line by a predetermined displacement during a write operation. Because of this, the preferred track following position is sometimes off the centerline of the track.
The servo preamble 42 provides information used to adjust the read channel electronics for reading and processing the positional servo information. The servo position portion 44 of the servo wedge provides the actual position data to be read by the read element 28 and used for positioning the head 22. The illustrated servo preamble 42 begins with a pre-burst gap 48 in which no transitions are recorded followed by an automatic gain control (AGC) field 50 that might include a regular pattern of transitions (e.g., a position 3T pattern followed by a negative 3T pattern) used to adjust the gain of the read channel electronics. The servo preamble next includes a sync pattern 52 for setting the clock in the read channel electronics when reading the servo positional information, which may be followed by a servo address mark 54 that indicates to the read channel electronics that the subsequent information will be servo positional information, as opposed to data. Next the servo preamble 42 may include an index field 56 that provides positional information within the track, i.e., whether the servo wedge is that designated as the first servo wedge on the track.
After the servo preamble 42 is the servo position information 44, including coarse position information 58 and fine position information 60-66. The coarse position information 58 may, for example, comprise Grey codes the numerically designate each of the tracks on the storage surface. Generally, a gap separates the coarse position information 58 and the finer track positioning information provided by servo bursts 60-66. The checkerboard pattern 60-66 of offset servo bursts A, B, C, D of recorded information are written to have precise and desired positions with respect to the centerlines of different tracks within a predetermined grouping of tracks. This allows the read element to generate a control signal related to the linear offset with respect to a desired position relative to a track, such as the track centerline, which control signal can be used to adjust the position of the head with respect to the track.
The illustrated checkerboard pattern consisting of the A, B, C, D servo bursts is formed by a servo writer using multiple write and erase passes during manufacture so that each of the servo wedges includes the illustrated pattern of four rectangular servo bursts repeated at desired radial and tangential positions. The servo bursts A, B, C, D might internally consist, for example, of a repeating 3T pattern, with the servo bursts surrounded by regions without recorded transitions. In the normal operation of the disk drive, the boundaries of the servo busts are detected in track seek and track following operations to periodically generate a position error signal (PES) that can be used to adjust the position of a head with respect to a data track. In between the servo bursts, multiple (typically 3-5) data blocks are stored along the track. The servo control mechanism works in cooperation with the buried servo information to place the head accurately at a desired position with respect to the track as the servo burst passes beneath the head. No additional positioning information is available until the next servo wedge passes by the head. Accordingly, the servo control mechanism attempts to hold the head in a fixed position with respect to the track position identified by the most recent servo burst. It is possible for the head or the disk to move due to mechanical impacts, vibrations, thermal variances or other disturbances in the system before reading the next servo burst.
In addition to the track identification information within the servo wedge, storage surfaces are sometimes provided with additional information to indicate when the desired track and sector has been located in a seek operation. An ID header block may optionally be provided between the servo burst and the first data block of a sector. The ID header primarily includes identification for the track and the following sector. Aspects of the use of synchronization patterns and headers are described, for example, in U.S. Pat. Nos. 5,541,783 and 5,796,534, which patents are hereby incorporated by reference. While header information may be provided at the start of sectors in many systems, other techniques for identifying tracks that do not use headers are known. For example, ID and header information can be included within the servo bursts as described in the article by Finch, et al., “Headerless Disk Formatting: Making Room for More Data,” Data Storage (April 1997), pp. 51-54, or servo information can cross-reference information stored in a corresponding table in memory as described in the IBM Storage publication by Hetzler, “No-ID Sector Format,” dated Jan. 8, 1996.
Following a servo wedge 40 (and ID header block when used), multiple blocks of data (typically 3-5) can be stored along a track, as shown in FIG. 10. Each block of data 70, 72, 74 includes a data synchronization pattern 76, 78, 80 positioned adjacent the data storage region of the block. Typically, a data block 70, 72, 74 is followed by an ECC block 82, 84 that stores error identifying and correcting codes for the preceding data block. The data storage region of each data block is typically of sufficient size to store data signals to represent 512 bytes of data.
The data synchronization pattern includes synchronization information that can be extracted to establish a sampling frequency and phase for recovering data from a data storage region. The conventional synchronization pattern 76, 78, 80 is written by the write element of a head in an operation in which the associated data block 70, 72, 74 is written. The clock rate used to write the synchronization pattern is also used to write the subsequent data blocks. During a subsequent read operation, the read element of a head passes over the synchronization pattern and detects a pattern of transitions (e.g., a 2T or 3T pattern) from which a clock is derived for reading the subsequent data blocks. Conventionally it is preferred that the synchronization pattern be substantially uniform in the radial direction, varying only in the tangential direction for a read element positioned in a desired manner with respect to the track. Typically, the disk control logic and the actual rotational speed of the disk determine the data rate written for the data synchronization pattern and the data storage region that follows. Accordingly, the actual data rate can vary from block to block and sector to sector and, consequently, the amount of space occupied by the stored data can change. To accommodate these changes, there is typically a gap (an interblcok gap) or data pad 86, 88 following each data block to insure that there is sufficient physical separation between successive blocks along a track to allow data blocks to be written without overwriting a subsequent block header or trailing servo burst.
There is a tension in designing data storage systems between increasing the track density, which typically requires denser servo wedge patterns, and loss of storage area due to the provision of increased densities servo information. It is desirable to provide additional servo information without reducing the area on the data storage surface devoted to the actual storage of data. In other words, it is desirable to increase the storage density without increasing the overhead necessary for accurately storing and retrieving information.
FIG. 11 illustrates the differences between precise positional control during a write operation and poorer positional control during a write operation, such as might occur for a data block displaced away from a sector servo wedge when the disk is subject to vibration. The figure shows a data block within a well-written track 90 where the written data is symmetrically disposed about the centerline of the track. The write width of the track (TW) is symmetrically disposed within the pitch allowed for the track TP. For a well written track 90 such as is illustrated, a read head centered on a track senses a read area 92 positioned within the data stored on the track with good margins between the read area 92 and the edges of the written track area. The track arrangement might, for example, illustrate a track pitch TP of 1.0 μm, a write width TW of 0.8 μm, and a read width 92 of 0.6 μm.
In contrast to the well-written track 90, the badly-written track 94 is subject to poor positional control and mechanical disturbance. The path of the badly-written track varies within the track pitch in an irregular manner. It is possible for such an irregular write path to be sufficiently misaligned that the read width is partially off of the written area, as indicated at 94. Such a misalignment reduces the quality of the read out data and can lead to read errors. To avoid this problem, it is conventional to increase the size of the write width relative to the read width. A different error, indicated at 96 in FIG. 11, occurs when a write head becomes misaligned to a sufficient extent to overwrite the written area of an adjacent track. Such an error is extremely undesirable and is avoided by increasing the track pitch.
Each of the potential errors illustrated in FIG. 11 is conventionally addressed by increasing the spacing between structures within the track. Because of this, improvements in head positioning accuracy can be significant factors in improving the density of tracks and written information.
The preceding discussion has set forth aspects of the servo and other informational structures provided on magnetic disk drives and how this information is used. With this background established, the discussion now returns to control systems for magnetic storage disk drives and how different observable variables are used in such systems. Commonly, the position error signals (PES) that are observed for controlling the position of the read and write head with respect to the track position are derived dedicated servo patterns. In particular, fine positional control is effected by deriving PES from the A, B, C, D servo burst patterns illustrated in FIGS. 9A & 9B. The position error signals detected by translating the read head over the A, B, C, D servo burst patterns vary linearly about zero and so provide both the magnitude and the direction of corrections to be made to the position of the head. Here the detected position error signals are the observable variables that can be used for controlling the position of the head with respect to a data track. As discussed above with reference to FIGS. 10 and 11, however, control in accordance with the conventional servo information is not entirely sufficient to meet present and anticipated needs for magnetic storage systems. Alternate control strategies are desirable.
U.S. Pat. No. 5,233,487 to Christensen, et al., relates to magnetic disk drives and the control of head position with respect to a track using an absolute value observable variable for the control system. The Christensen patent describes a disk drive that periodically interrupts data reading and writing operations to perform a calibration of the optimal head offset with respect to a data track for data writing operations. The calibration proceeds by translating a read head across the track through a range of intentional mislaingments with respect to the track. Data are read in the deliberately misaligned positions and the errors that occur in reading data from the track are obtained as a function of misaligned position. Error rates are derived by detecting blocks of data and then applying the standard data block error correction using the ECC circuitry of the disk drive with reference to the ECC data encoded with the data stored on the disk. The disk drive detects at what track position, and hence what level of intentional misalignment, that the observed error rate exceeds certain target error rates having a known association with known track positions. This allows the characteristics of the operating disk drive to be compared to known standards.
It should be noted that the target error rates (indicated in FIG. 2 of the Christensen patent) are absolute value functions. As such, the target error rates that are derived in the Christensen patent's system do not indicate the direction in which the head is misaligned with respect to the data track. The sign of the misalignment is known by the control system because of the intentional positioning of the read head, much in the same way that dither can be used to generate a known direction for a correction within an absolute value observable control system.
The track positions detected using the Chritensen patent's method can be used to calculate a track center position, recalibrate the relative motion achieved by application of a controlled displacement voltage, and establish an optimal offset for reading data from and writing data to the track. In this way the Christensen patent performs a servo control function using information other than the servo wedge and servo burst information illustrated in FIGS. 9A & 9B. In particular, the Christensen patent performs servo control using error information derived from constraints associated with blocks of data stored on the disk. The Christensen patent's method, however, does not allow derivation of position error signals on a continuous basis or while data are being extracted from the magnetic storage disk.
The Christensen patent performs servo functions using an observable variable, a bit error rate derived from ECC operations on data blocks, that provides only absolute value information relating to the positional error. On the other hand, practice of the Christensen patent's method requires use of a head to track displacement that shares many problems with dithering and so cannot be used as data are gathered. It is desirable to provide a method and system capable of using absolute value information to perform a control function.